Electricity generated from clean and renewable sources, such as water, wind, or sunlight, can be successfully converted to electrical energy if the generated electrical energy can be storage efficiently stored and distributed by high capacity secondary batteries. In this regard, rechargeable ion batteries have attracted global attention in the expansion of clean and renewable energy research. Multivalent ion batteries are being increasingly explored as an alternative to lithium-ion batteries (LIBs) which can be damaged owing to their tendency to form dendrites at the anode at a high rate. Given the potential of multivalent ions to yield more than one electron transfer for each redox reaction, their use is likely to result in batteries having high capacity and fast charge ability for energy storage. However, multivalent ions, such as Ni2+, Zn2+, Mg2+, Ca2+, Ba2+ ions, which have been explored for battery systems that operate with divalent charge, come with their own set of challenges, such as formulating an electrolyte capable of reversible plating, low operating voltage, and lack of cathode compatibility.
Electrolyte development is one of the major challenges in the development of a secondary Mg ion battery design. Halogen containing magnesium electrolytes, such as Grignard reagents (R—Mg—X, where R is an organic residue and X is halogen), have achieved reversible Mg deposition and dissolution, however, their widespread use is impaired by the corrosive nature of the halogen, and the corrosive nature can cause damage to the electrolyte cell components, such as the current collectors. Additionally, halogen containing electrolytes are not known to be compatible with cathodes other than low voltage Chevrel-type cathodes.
Electrolytes based upon magnesium bis(trifluoromethylsulfonylimide) (“Mg(TFSI)2”) have also drawn considerable interest not only due to its simplicity, but also its anodic stability from the conjugated TFSI anion structure, yet there are a number of challenges associated with its use. For example, electrolytes containing a tetrahydrofuran (THF) solution of magnesium bis (trifluoromethylsulfonylimide) and magnesium chloride (Mg(TFSI)2/MgCl2) require a strong Lewis acid such as AlCl3. However, the AlCl3 reacts toward THF, and the generated TFSI anions can potentially lead to the decomposition of the magnesium anode surface. Mg+2 in glymes have proven to intercalate with oxides cathode, namely V2O5, V2O5.xH2O, although these systems do not exhibit sufficient columbic efficiency to be found acceptable for large scale applications.
Synthetic strategies to develop Mg electrolytes which can achieve improved reversible Mg deposition while overcoming concerns related to corrosion and limited cathode compatibility are, therefore, needed.